The present invention relates to a device for detecting a deterioration of a catalyst mounted on an engine to detect the deterioration by two air-fuel ratio sensors provided upstream and downstream sides of a catalytic converter and a method therefor.
Recently, in the feedback control of an air-fuel ratio of an engine, a double O.sub.2 sensor system where air-fuel ratio sensors such as O.sub.2 sensors are provided upstream and downstream sides of a catalytic converter for purifying exhaust gas of the engine so that an irregularity of output characteristics of the first O.sub.2 sensor of the upstream side is compensated by the second O.sub.2 sensor of the downstream side to improve a decrease of a controlling accuracy due to the irregularity of the output characteristics of a single O.sub.2 sensor has been proposed.
In the double O.sub.2 sensor system, a deterioration of a catalyst is detected by comparing the output characteristics of the two O.sub.2 sensors. For example, Japanese Utility Model Application Laid-Open 63-128221 discloses a technique for determining a deterioration of a catalyst based upon a ratio of inverting frequencies (or periods) of outputs of first and second air-fuel ratio sensors provided upstream and downstream sides of a catalytic converter.
In general, purifying performance of a catalyst has a strong correlation with a storage effect of oxygen contained in the catalyst. As shown in FIG. 10(a), the catalyst before a deterioration has a preferable storage effect of oxygen, and an output FVO.sub.2 of a first O.sub.2 sensor upstream side of a catalytic converter is deflected toward rich and lean with time elapsed by means of an air-fuel ratio feedback correction, whereas an output RVO.sub.2 of a second O.sub.2 sensor downstream side of the catalytic converter is substantially constant, but if the catalyst is deteriorated, its storage effect is deteriorated, and, as shown in FIG. 10(b), the output RVO.sub.2 of the second O.sub.2 sensor downstream side of the converter is deflected toward rich and lean with time elapsed at a predetermined delay from the output FVO.sub.2 of the first O.sub.2 sensor upstream side of the converter.
More particularly, when the catalyst is deteriorated, the output characteristics of the second O.sub.2 sensor downstream side of the catalytic converter approach those of the first O.sub.2 sensor upstream side of the converter. Thus, a difference of inverting frequencies of both the first and second O.sub.2 sensors is reduced and hence a deterioration of the catalyst can be decided by comparing a ratio of the inverting frequencies of both the first and second O.sub.2 sensors with a set value.
In this connection, the inverting frequencies of the outputs of the first and second O.sub.2 sensors provided upstream and downstream sides of the catalytic converter can be calculated from the number of times of being cut or crossed by the outputs of the first and second O.sub.2 sensors across upper and lower slice levels provided at the first and second O.sub.2 sensors within a predetermined time. As the upper and lower slice levels of the first and second O.sub.2 sensors, predetermined constant values unitarily defined previously as shown in FIGS. 9(a) to 9(c), are normally used.
However, when a temperature of exhaust gas of an engine is varied with a variation of operating conditions of the engine such as an engine speed and a vehicle speed even if a load state of the engine such as a basic fuel injection quantity and the like is the same as that after or before the catalyst is deteriorated, the output amplitude of the second O.sub.2 sensor downstream side of the catalytic converter is remarkably changed in case where the catalyst is deteriorated.
Accordingly, the inverting frequencies of the outputs of the first and second O.sub.2 sensors are varied in dependency on the operating condition of the engine, and when a vehicle is traveled, for example, at a high speed in which the engine of the vehicle rotates at a high speed and a temperature of exhaust gas of the engine rises, the amplitude of the output RVO.sub.2 of the second O.sub.2 sensor is reduced, as shown in FIG. 11, Thus, if the unitarily defined constant slice levels are employed, the output RVO.sub.2 of the second O.sub.2 sensor does not cut the slice level of the O.sub.2 sensor even if the amplitude of the output RVO.sub.2 of the second O.sub.2 sensor is produced due to a deterioration of the catalyst. Then, a difference of the inverting frequency of the output of the first O.sub.2 sensor upstream side of the catalytic converter and the inverting frequency of the output of the second O.sub.2 sensor downstream side of the converter is increased. Hence even if the catalyst is deteriorated, the catalyst is judged to be normal, and an erroneous decision occurs in the case of diagnosing a deterioration of the catalyst.